The combination of an Insulated Gate Bipolar Transistor (IGBT) and an anti-parallel diode is a circuit that sees use in many power electronics circuits. The diode often used is referred to as a Fast Recovery Diode (FRD) or a Fast Recovery Epitaxial Diode (FRED). The diode is coupled between the collector and the emitter of the IGBT in the anti-parallel direction. The IGBT may be a Punch Through type IGBT (PT IGBT) or a Non-Punch Through type IGBT (NPT IGBT).
FIG. 1 (Prior Art) is a simplified diagram of one such application of an IGBT/diode combination circuit. The circuit is a three-phase motor drive circuit 1 that has three phase legs 2, 3 and 4. Each phase leg is coupled between a first DC voltage conductor 5 and a second DC voltage conductor 6. In the illustrated example, a 300 volt DC voltage is present between conductors 5 and 6. The battery symbol 29 is not typically a battery. In the specific circuit illustrated here, symbol 29 represents a three-phase AC rectification circuit that converts three-phase AC power into a 300 volt DC voltage. Each of the three legs 2, 3 and 4 includes two combination switches as illustrated. One of the combination switches of the leg is for coupling an output node of the leg to the first DC voltage conductor 5, whereas the other combination switch of the leg is for coupling the output node of the leg to the second DC voltage conductor 6. Leg 2 includes combination switches 7 and 8. Leg 3 includes combination switches 9 and 10. Leg 4 includes combination switches 11 and 12. The IGBTs are designated with reference numerals 14-19, and the diodes are designated with reference numerals 20-25.
The three windings of a motor 13 are coupled to the corresponding three output nodes r, g and b of the three legs. The gates of the IGBTs are driven as known in the art to rotate the motor as desired. The ON/OFF duty cycles of the IGBTs determine the time averaged voltages at the output nodes r, g and b. In order to prevent the circuit from generating audible noise, the switching frequency of the IGBTs is generally made to be higher than 15 kHz. In many cases, the motor windings are so highly inductive that the output currents change very slowly compared to the >15 kHz switching frequency. The motor load on a leg can therefore be considered to be a constant current source as compared to the switching on and off of the IGBT.
FIG. 2 (Prior Art) is a diagram of leg 2 with the inductive load of motor 13 being represented as a current source 26. Prior to IGBT 15 being turned on, current flows from current source 26, to output node r, and then up through diode 20, and to conductor 5. IGBT 14 is off. When IGBT 15 is turned on and starts flowing current, there is a corresponding decrease in the current flow through diode 20. If the rate of increase in current through IGBT 15 is high, then voltage and current spikes can result.
FIG. 3 (Prior Art) is a waveform diagram that illustrates ringing that might occur when IGBT 15 of FIG. 2 is turned on. Initially, the current from modeled current source 26 passes through node r, then through forward biased diode 20, and up to conductor 5. Then, at time T1, the lower IGBT 15 is turned on. The total amount of current being supplied onto node r remains substantially constant due to current source 26. Therefore, when current begins flowing through lower IGBT 15, there is a commensurate decrease in current flow through upper diode 20. If the rate of change of current flow through the lower IGBT is rapid (a large negative dI/dt), then the corresponding high rate of falling current in the diode 20 may result in overshoot and a negative current spike 27.
The rapid decrease in the current in the upper leg can result in a corresponding voltage spike across the upper IGBT as indicated by the upper waveform. If the decrease in diode current in the upper leg is rapid enough, then the voltage drop across the upper diode 20 may be so large that diode 20 will experience avalanche breakdown. In the diagram of FIG. 3, the flat bottom portion 28 of the first voltage oscillation after time T1 in the upper waveform is due to avalanche breakdown of diode 20. The subsequent high amplitude ringing of the voltage on node r damps out as illustrated, but the short duration of the high voltage ringing is problematical. First, many designers of power electronics circuits may simply not want to design products that operate with such large voltages. Their reasons may involve reliability concerns and/or safety concerns. Second, the high amplitude voltage ringing may result in undesired amounts of electromagnetic radiation being emitted. This unwanted emitted electromagnetic radiation causes Electro-Magnetic Interference (EMI). The rate at which current flow through diode 20 is made to decrease may be reduced to prevent such high voltage ringing on node r, but decreasing the rate of current change through IGBT 15 translates into increased energy losses. It is desired to be able to switch the IGBT quickly for efficiency reasons, and yet to produce no large voltage spikes and to emit little electromagnetic radiation.